1. Field of the Invention
The present invention relates to a plasma CVD apparatus and method suitable for depositing an amorphous silicon thin film utilizable for solar cells or thin film transistors and the like.
2. Description of the Related Art
An apparatus disclosed in JP-A-5-343338, for example, has been known as a conventional plasma CVD apparatus used for manufacturing large-sized solar cells by depositing an amorphous silicon thin film onto a large-sized substrate. The plasma CVD apparatus has a cathode electrode unit, and an anode electrode on which a substrate is loaded. The cathode electrode unit has a plurality of cathode electrodes which are arranged so that all of their lower surfaces are positioned on a common plane. The cathode electrode unit has a plasma non-generation region at the backs of the plurality of cathode electrodes. Gas ejection openings of a plurality of gas supply tubes for supplying a material gas toward a substrate are arranged in the plasma non-generation region. Further, the cathode electrode unit has a plurality of gas evacuation openings, each of which is positioned between the two adjacent gas ejection openings. The gas ejection openings and the gas evacuation openings are arranged by turns. Since the plurality of gas ejection openings are placed in the plasma non-generation region, generation of contamination powder can be prevented. This arrangement such that the plurality of gas ejection openings and the gas evacuation openings ire placed to be mutually separated causes the material gas to diffuse uniformly.
As other related conventional plasma CVD apparatus, there are apparatus disclosed in the laid-open publications of JP-A-4-236781 and JP-A-7-330488, for example. The plasma CVD apparatus of JP-A-4-236781 has an electric discharge electrode formed by a plane coil having a ladder shape, which is placed to be parallel with a substrate. The ladder-shaped plane coil is formed out of a wire rod. A material gas is introduced into a reactor through a gas introduction tube arranged in one spot of the reactor while the reactor is evacuated through a gas evacuation tube arranged in another spot of the reactor. The plane coil can enhance strength of an electric field and improve uniformity thereof. The plasma CVD apparatus of JP-A-7-330488 has a structure such that a hollow ladder electrode with an earth shield is arranged to be parallel to a substrate. This structure can make the region of strong electromagnetic field strength in the vicinity of the electrode, into which a reactive gas is introduced, and thereby enhance a quality of the amorphous silicon thin film deposited on the substrate. Also, an evacuation tube for evacuating the inside of the reactor is placed in the surrounding area of the substrate in the reactor.
When depositing the amorphous silicon thin film onto a rectangular substrate having large area (for example, 550 mm.times.650 mm) in order to make a large-area solar cell by making use of the conventional ordinary plasma CVD apparatus, following problems should be posed.
When attempting to form the thin film at a high rate by using the conventional plasma CVD apparatus, great electric power must be supplied. The supply of the great electric power causes the film quality to be deteriorated. This deterioration of the film quality is due to a higher silane gas generated within high density plasma. Therefore, it is necessary to immediately remove this higher silane gas from a reactive region during the thin film deposition.
On the contrary, in the conventional plasma CVD apparatus, a gas supply section through which the material gas is introduced into the internal space of the reactor is generally the shower-head-type gas supply mechanism and functions as one of two electrodes forming a parallel-plate-type electrode structure. The gas supply section is disposed in front of the processed surface of the substrate and supplies the material gas toward the processed surface of the substrate. Also, an evacuation mechanism for causing the inside of the reactor to be vacuum state or evacuating unnecessary gases within the reactor to the outside is arranged at the peripheral section of the reactor around the substrate. Therefore, in the conventional plasma CVD apparatus, the material gas streams from the center region to the peripheral region on the substrate surface to be processed, and thus the gas upper stream region is formed at the center of the processed surface of the substrate while the gas lower stream region is formed at the periphery of the processed surface. This matter in accordance with the conventional plasma CVD apparatus resulted in the difficulty of immediately removing the higher silane gas from the reactive region and posed the problem such that the film quality at the center of the surface is easy to be deteriorated. The area of the substrate becomes wide the more, the more remarkable is this problem. The thin film utilized for the solar cell is especially required to have a high quality in the aspect of the film structure. The conventional plasma CVD apparatus has difficulty of no satisfying the requirements because it has the above-mentioned problems.
Also, in the respective plasma CVD apparatus mentioned above in accordance with the laid-open publications of Japanese application, concerning the flowing of the gas introduced toward the substrate, the upper stream region and the lower stream region are formed on the basis of the positional relationship of gas discharge openings and gas evacuation openings and therefore the similar problem as mentioned above is raised.